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$20 WiFi-enabled IoT module runs FreeRTOS on Cortex-M4

Apr 21, 2016 — by Eric Brown — 2,825 views

MediaTek’s $20 “LinkIt 7687 HDK” module runs FreeRTOS on its WiFi-enabled, Cortex-M4F based MT7687F SoC, and offers Arduino shield expansion.

LinkIt Smart 7688

Readers of LinuxGizmos are likely familiar with the MediaTek Labs LinkIt family of IoT-focused, open spec hacker boards from its only Linux-friendly board, the $13, OpenWrt-based LinkIt Smart 7688. Now that we’ve transitioned from LinuxGizmos to HackerBoards, we’re starting to explore some MCU-based boards for real-time operating systems, such as the new LinkIt 7687 HDK, the first of a family of MediaTek LinkIt Development Platform for RTOS boards.

Designed to run the open source FreeRTOS, this latest LinkIt is the first to come in the form of a full hardware development kit (HDK), says MediaTek. While the other LinkIt boards, including some other RTOS-driven models (see farther below) are stripped-down SBCs similar to computer-on-modules, and ready to slot into commercial devices, the LinkIt 7687 HDK is more of a development and prototyping board.

LinkIt 7687 HDK, with and without heat sink on the MT7687F SoC
(click images to enlarge)

Developed by Silicon Application Corp. (SAC), using a MediaTek board reference design, the HDK integrates an 8-pin Arduino Uno Rev 3 shields expansion connector, and measures 109 x 61mm. By comparison, the 56 x 26mm LinkIt Smart 7688 requires an optional breakout board to support Arduino shield expansion.

The LinkIt Development Platform for RTOS integrates the MediaTek LinkIt SDK v3, offering a common tool chain and set of APIs for multiple chipsets. The platform is designed for building advanced connected appliances, home and office automation devices, smart gadgets, and other IoT innovations, says MediaTek. Future HDKs will enable developers to target other IoT segments with different wireless options, such as Bluetooth and cellular, says the company.

Like the LinkIt 7687 HDK, future HDKs in the LinkIt Development Platform for RTOS series will all run FreeRTOS on SoCs based on ARM Cortex-M4 MCUs. Source code is available for modules implementing TCP/IP, SSL/TLS, HTTP (client and server), SNTP, DHCP daemon, MQTT, XML, and JSON technologies. Keil μVision and various command-line tools are available for development and debugging.

With the LinkIt 7687 HDK and MediaTek MT7687F SoC, MediaTek appears to be shifting away from its own LinkIt OS running on various exotic hybrid chips and toward widely used industry standards: FreeRTOS and Cortex-M4.

MediaTek MT7687F SoC

The LinkIt 7687 HDK is built around the MediaTek MT7687F, a WiFi-enabled, Cortex-M4F based SoC. The 8 x 8mm QFN-packaged SoC is equipped with 352KB embedded SRAM, as well as 352KB ROM and 2MB serial flash. The limited RAM and lack of external memory expansion likely prevent the HDK from running small-footprint Linux variants such as uClinux. The SoC’s 802.11b/g/n subsystem features AES and 3DES/SHA security. The Cortex-M4F processor is a variant of the Cortex-M4 that includes the M4’s optional single-precision FPU (floating point unit).

MT7687F SoC simplified and detailed block diagrams
(click images to enlarge)

Although the MT7687F name is similar to the 580MHz MediaTek MT7688AN SoC found on the OpenWrt-ready LinkIt Smart 7688, the latter is a more advanced, 580MHz MIPS-based SoC, much like a Qualcomm Atheros AR9331. MediaTek also offers a LinkIt Connect 7681 development platform based on its sub-100MHz, Bluetooth-focused MT7681 SoC. MediaTek’s MT7681 uses a simpler RISC-based MCU rather than an ARM Cortex-M4. (FreeRTOS supports both the Cortex-M4 and the -M4F.)

The original LinkIt One board announced back in 2014 ran on an intermediate, 260MHz, ARMv7 SoC called the Aster (MT2502). Although this is a step up from the MT7681, it runs the LinkIt OS RTOS instead of Linux. The LinkIt One was chosen by Amazon for one of its Seeed Grove-enhanced AWS IoT Starter Kits for the Amazon Web Services (AWS) IoT platform. The wearable-focused Aster MT2502 also appears in the LinkIt Assist 2502 development platform.

LinkIt 7687 HDK dev board details

The LinkIt 7687 HDK with LinkIt Development Platform for RTOS ships with the 192MHz MT7687F SoC. The 109 x 61mm HDK board weighs 25 grams, and supports industrial, -40 to 85°C conditions.

Other LinkIt 7687 HDK features include:

  • Arduino Uno (Rev 3) compatible 8-pin extension connector with UART, I2C, SPI, I2S, PWM, IrDA and ADC
  • Micro-USB for debug (CMSIS-DAP) and power (VBus, 5 V)
  • External VIN (1.8 to 3.2V) input and battery (2x AA) support
  • 6x customizable LEDs for UART and power
  • 3x push buttons for reset, RTC interrupt, and external interrupt
  • XTAL (Crystal Oscillator) — 40MHz clock with idle mode, 32.768kHz clock for RTC or external modes
  • Mass Storage Device (MSD) flash programming interface
  • USB connectors/interfaces supporting re-enumeration
  • Virtual COM port UART via USB for PC connections
  • Printed circuit board antenna with U.FL connector for conducted testing
  • Reserved headers for power consumption (current) measurement

Further information

The LinkIt 7687 HDK, the first of the LinkIt Development Platform for RTOS offerings, is available for $20, including the MediaTek MT7687F SoC module, which sells for $5 separately. More information may be found at the MediaTek Labs LinkIt 7687 HDK and LinkIt Development Platform for RTOS web pages.

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